Indium or tin bonded acoustic transducer systems

ABSTRACT

An acoustic transducer comprised of a resonator, an acoustic energy generating means, such as one or more piezoelectric crystals, and a bonding layer comprised of indium or tin for attaching the acoustic energy generating means to the resonator. The resonator comprises a material selected from the group consisting of quartz, sapphire, silicon carbide, silicon nitride, aluminum, ceramics and stainless steel. The resonator may form a detachable part of a container or the acoustic energy generating means may be attached directly to a side or to the bottom of the container, in which case a part of the container acts as the resonator.

This application is a continuation of application Ser. No. 10/185,917,filed Jun. 27, 2002, which is a continuation-in-part of U.S. Pat. No.6,722,379, which is a continuation-in-part of U.S. Pat. No. 6,222,305,which is a continuation-in-part U.S. Pat. No. 6,188,162.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to megasonic cleaning systems having achemically inert resonator and a piezoelectric crystal and moreparticularly to a system in which the crystal is bonded to the resonatorusing tin or indium.

2. Background Information

It is well-known that sound waves in the frequency range of 0.4 to 2.0megahertz (MHZ) can be transmitted into liquids and used to cleanparticulate matter from damage sensitive substrates. Since thisfrequency range is predominantly near the megahertz range, the cleaningprocess is commonly referred to as megasonic cleaning. Among the itemsthat can be cleaned with this process are semiconductor wafers invarious stages of the semiconductor device manufacturing process, diskdrive media, flat panel displays and other sensitive substrates.

Megasonic acoustic energy is generally created by exciting a crystalwith radio frequency AC voltage. The acoustical energy generated by thecrystal is passed through an energy transmitting member and into thecleaning fluid. Frequently, the energy transmitting member is a wall ofthe vessel that holds the cleaning fluid. The crystal and its relatedcomponents are referred to as a megasonic transducer. For example, U.S.Pat. No. 5,355,048, discloses a megasonic transducer comprised of apiezoelectric crystal attached to a quartz window by several attachmentlayers. The megasonic transducer operates at approximately 850 KHz.Similarly, U.S. Pat. No. 4,804,007 discloses a megasonic transducer inwhich energy transmitting members comprised of quartz, sapphire, boronnitride, stainless steel or tantalum are glued to a piezoelectriccrystal using epoxy.

It is also known that piezoelectric crystals can be bonded to certainmaterials using indium. For example, U.S. Pat. No. 3,590,467 discloses amethod for bonding a piezoelectric crystal to a delay medium usingindium where the delay medium comprises materials such as glasses, fusedsilica and glass ceramic.

A problem with megasonic transducers of the prior art is that theacoustic power that can be generated by the megasonic transducer in thecleaning solution is limited to about 10 watts per cm² of activepiezoelectric surface without supplying additional cooling to thetransducer. For this reason, most megasonic power sources have theiroutput limited, require liquid or forced air cooling or are designed fora fixed output to the piezoelectric transducer or transducers.Typically, fixed output systems are limited to powers of 7-8 watts/cm².This limits the amount of energy that can be transmitted to the cleaningsolution. If more power is applied to the transducer, the crystal canheat up to the point where it becomes less effective at transmittingenergy into the cleaning solution. This is caused either by nearing themaximum operating temperature of the crystal or, more often, by reachingthe failure temperature of the material used to attach the crystal tothe energy transmitting means.

Another problem with prior art cleaning systems that utilize megasonictransducers, is that there is no practical way of replacing a defectivetransducer once the transducer has been attached to the cleaning system.This means that users have to incur large expenses to replace defectivetransducers, for example by purchasing a whole new cleaning vessel.

SUMMARY OF THE PRESENT INVENTION

Briefly, the present invention is a megasonic cleaning system comprisedof a resonator, one or more piezoelectric crystals and a tin or indiumbonding layer for attaching the piezoelectric crystal (or crystals) tothe resonator. The resonator comprises a material selected from thegroup consisting of quartz, sapphire, silicon carbide, silicon nitride,aluminum, ceramics and stainless steel.

The resonator may comprise a strip resonator or a one-piece cleaningtank. In the case of a one-piece cleaning tank, the piezoelectriccrystal (or crystals) is attached directly to the sides or bottom of thetank. In the case of a strip resonator, the crystals are attached to thestrip resonator, and the resonator is incorporated into a part of acleaning tank.

The piezoelectric crystal (or crystals) is capable of generatingacoustic energy in the frequency range of 0.4 to 2.0 MHz when power isapplied to the crystal. The attachment layer is comprised of indium andis positioned between the tank and the piezoelectric crystal so as toattach the piezoelectric crystal to the tank. A first adhesion layercomprised of chromium, copper and nickel is positioned in contact with asurface of the piezoelectric crystal. A first wetting layer comprised ofsilver is positioned between the first adhesion layer and the bondinglayer for helping the bonding layer bond to the first adhesion layer.

One way of attaching the crystal to the resonator involves the use of acombination layer. This is especially useful when the resonator is asingle piece tank. The combination layer is applied to the region of thetank or resonator to which the crystal is to be attached. Thecombination layer helps the indium or tin bonding layer bond to the tankor resonator, and preferably comprises a silver conductive emulsion(paste) that is applied to the tank or resonator using a screen printprocess.

Alternatively, the combination layer can be replaced by a secondadhesion layer and a second wetting layer. The second adhesion layer iscomprised of chromium, copper and nickel and is positioned in contactwith a surface of the tank. The second wetting layer is comprised ofsilver and is positioned between the second adhesion layer and thebonding layer for helping the bonding layer bond to the second adhesionlayer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an acoustic transducer assemblyaccording to the present invention;

FIG. 2 is a side view of an acoustic transducer according to the presentinvention;

FIG. 3 is side view of a spring/button electrical connector boardaccording to the present invention;

FIG. 4 is an exploded view of the acoustic transducer assembly accordingto the present invention;

FIG. 5 is a side view of an acoustic transducer according to the presentinvention;

FIG. 6 is an exploded view of a megasonic cleaning system according tothe present invention;

FIG. 7 is a schematic circuit diagram of the present invention;

FIG. 8 is an exploded view of a single-piece tank megasonic cleaningsystem according to the present invention;

FIG. 9 is a side view of an acoustic transducer in which a one-piecetank is the resonator; and

FIG. 10 is a side view of an acoustic transducer in which the resonatorcan be a one-piece tank or a strip resonator according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a cross section of an acoustic transducer assembly 10comprised of an acoustic transducer 14, a spring/button electricalconnector board 18 and a housing 22. The transducer 14 comprises aresonator 26 which is bonded to a piezoelectric crystal 30. Theelectrical connector board 18 comprises a printed circuit board (PCB) 34which has a plurality of first spring/button connectors 38 and aplurality of second spring/button connectors 42 connected to it. Thehousing 22 is a case that encloses the electrical connector board 18 sothat it is protected from the environment. The electrical connectorboard 18 and the acoustic transducer 14 sit in a cavity 46 inside thehousing 22.

The resonator 26 forms part of a wall in the housing 22 that covers andseals the cavity 46. A surface 50 of the resonator 26 forms an externalside of the acoustic transducer assembly 10. In the preferredembodiment, the acoustic transducer 14 is used to generate megasonicacoustic energy in a cleaning apparatus used to clean semiconductorwafers. The surface 50 will be in contact with the cleaning fluid usedin the cleaning apparatus.

FIG. 2 illustrates that the acoustic transducer 14 comprises thepiezoelectric crystal 30 attached to resonator 26 by a bonding layer 60.In the preferred embodiment, a plurality of other layers are disposedbetween the piezoelectric crystal 30 and the resonator 26 to facilitatethe attachment process. Specifically, a first metal layer 64 is presentadjacent to a front surface 68 of the bonding layer 60. A second metallayer 72 is present adjacent to a back surface 76 of the bonding layer60. A blocking layer 80 is positioned between the metal layer 72 and thepiezoelectric crystal 30 to promote adhesion. In the preferredembodiment, the blocking layer 80 comprises a chromium-nickel alloy, andthe metal layers 64 and 72 comprise silver. The blocking layer 80 has aminimum thickness of approximately 500 Å and the metal layer 72 has athickness of approximately 500 Å.

In one embodiment, the piezoelectric crystal 30 is comprised of leadzirconate titanate (PZT). However, the piezoelectric crystal 30 can becomprised of many other piezoelectric materials such as barium titanate,quartz or polyvinylidene fluoride resin (PVDF), as is well-known in theart. In one embodiment, two rectangularly shaped PZT crystals are usedin the transducer 14, and each PZT crystal is individually excited.

A blocking/adhesion layer 84 separates the metal layer 64 from theresonator 26. In the preferred embodiment, the blocking/adhesion layer84 comprises a layer of nickel chromium alloy which is approximately 500Å thick. However, other materials and/or thicknesses could also be usedas the blocking layer 84. The function of the blocking layer 84 is toprovide an adhesion layer for the metal layer 64. In the preferredembodiment, the metal layer 64 comprises silver and has a thickness ofapproximately 500 Å. However, other metals and/or thicknesses could beused for the metal layer 64. The function of the metal layer 64 is toprovide a wetting surface for the molten indium.

An additional layer is also disposed on a back side of the piezoelectriccrystal 30. Specifically, a metal layer 86 is positioned on the backside of the piezoelectric crystal 30 and covers substantially all of thesurface area of the back side of the crystal 30. Generally, the layer 86is applied to the piezoelectric crystal 30 by the manufacturer of thecrystal. The layer 86 functions to conduct electricity from a set of thespring/button connectors shown in FIG. 1, so as to set up a voltageacross the crystal 30. Preferably, the metal layer 86 comprises silver,nickel or another electrically conductive layer.

In the preferred embodiment, the bonding layer 60 comprises pure indium(99.99%) such as is commercially available from Arconium or Indalloy.However, indium alloys containing varying amounts of impurity metals canalso be used. The benefit of using pure indium and its alloys is thatindium possesses excellent shear properties that allow dissimilarmaterials with different coefficients of expansion to be attachedtogether and experience thermal cycling without damage to the attachedmaterials. Similarly, when tin is used as the bonding layer 60, it ispreferable to use pure tin (99.99% tin). However, tin containing varyingamounts of impurities, including other metals, can also be used.

In the preferred embodiment, the resonator 26 is a piece of sapphire(Al₂O₃). Preferably, the sapphire is high grade having a designation of99.999% (5,9s+purity). However, other materials, such as stainlesssteel, tantalum, aluminum, silica compounds (including quartz), ceramicsand plastics, can also function as the resonator 26. The purpose of theresonator 26 is to separate (isolate) the piezoelectric crystal 30 fromthe fluid used in the cleaning process, so that the fluid does notdamage the crystal 30. Thus, the material used as the resonator 26 isusually dictated, at least in part, by the nature of the fluid. Theresonator 26 must also be able to transmit the acoustic energy generatedby the crystal 30 into the fluid. Sapphire is a desirable material forthe resonator 26 when the items to be cleaned by the megasonic cleaningapparatus require parts per trillion purity. For example, semiconductorwafers require this type of purity.

In the preferred embodiment, the resonator 26 has a thickness “e” whichis preferably a multiple of one-half of the wavelength of the acousticenergy emitted by the piezoelectric crystal 30, so as to minimizereflectance problems. For example, “e” is approximately six millimetersfor sapphire and acoustic energy of about 925 KHz.

FIG. 3 illustrates the spring/button electrical connector board 18 inmore detail. Each first spring/button connector 38 comprises an uppersilver button 90 and a lower silver button 94. The upper silver button90 and the lower silver button 94 are attached to a plated silver spring98 and soldered to the printed circuit board (PCB) 34 so that theconnector 38 can provide an electrical connection to the acoustictransducer 14. The upper silver button 90 has a thickness “t” of about0.15 inches.

Similarly, each second spring/button connector 42 comprises an uppersilver button 98 and a lower silver button 102. The upper silver button98 and the lower silver button 102 are attached to a silver platedspring 106 and soldered to the PCB 34 so that the connector 42 canprovide an electrical connection to the acoustic transducer 14. Theupper silver button 98 has a thickness “r” of about 0.10 inches.Generally, the thickness “t” is greater than the thickness “r” becausethe first spring/button connector 38 has extend farther up to makecontact with the acoustic transducer 14 than does the secondspring/button connector 42 (see FIGS. 1 and 2).

A radio frequency (RF) generator provides a voltage to the PCB 34. ThePCB 34 includes electrical connections to the spring/button connectors38 and 42 so that the polarity of the spring/button connectors 38 ispositive and the polarity of the spring/button connectors 42 isnegative, or vice versa. Examination of FIG. 2 shows that in theacoustic transducer 14, the layers 26, 84 and 64 have a greater length“j” than the length “k” of the layers 60, 72, 80, 30 and 86. Thiscreates a step-region 110 on the silver layer 64 that can be contactedby the upper buttons 90 of the spring/button connectors 38. The upperbuttons 98 of the spring/button connectors 42 make electrical contactwith the silver layer 86.

The purpose of the spring/button connectors 38 and 42 is to create avoltage difference across the piezoelectric crystal 30 so as to exciteit at the frequency of the RF voltage supplied by the RF generator. Theconnectors 38 connect the metal layer 64 to the RF generator. Theconnectors 42 connect the layer 86 to the RF generator. The RF generatordelivers a RF alternating current to the piezoelectric crystal 30 viathe connectors 38 and 42. Preferably, this is a 925 KHz signal, at 600watts of power. The effective power in the piezoelectric crystal 30 isapproximately 15.5 watts/cm². The effective power in the piezoelectriccrystal 30 is defined as the forward power into the crystal 24 minus thereflected power back into the RF generator. Thus, the step-region 110,and the spring/button connectors 38 and 42, allow a voltage to be set upacross the piezoelectric crystal 30 without the need for solderingdiscrete leads to the layers 64 and 86.

In FIG. 3, a plurality of electrical components 114, such as capacitorsand/or inductors, are shown. These are used to balance the impedancebetween the RF input and the spring output.

FIG. 4 illustrates the way the acoustic transducer 14, the spring/buttonelectrical connector board 18 and the housing 22 fit together to formthe acoustic transducer assembly 10.

The acoustic transducer 14 (shown in FIG. 2) is prepared as follows(using the preferred materials described previously): Assuming that theresonator 26 is sapphire, the surface of the sapphire that will beadjacent to the layer 84 is cleaned by abrasive blasting or chemical orsputter etching. The blocking/adhesion layer 84 is then deposited on theresonator 26 by physical vapor deposition (“PVD”), such as argonsputtering. A plating technique could also be used. The silver layer 64is then deposited on the chromium blocking/adhesive layer 84 using argonsputtering. A plating technique could also be used.

The piezoelectric crystal 30 may be purchased with the layer 86 alreadyapplied to it. The blocking layer 80 and the metal layer 72 aredeposited on the crystal 30 by plating or physical vapor deposition.

If indium is used as the bonding layer 60, the resonator 26 and thepiezoelectric crystal 30 are both heated to approximately 200° C.,preferably by placing the resonator 26 and the crystal 30 on a heatedsurface such as a hot-plate. When both pieces have reached a temperatureof greater than 160° C., solid indium is rubbed on the surfaces of theresonator 26 and the crystal 30 which are to be attached. Since pureindium melts at approximately 157° C., the solid indium liquefies whenit is applied to the hot surfaces, thereby wetting the surfaces withindium. It is sometimes advantageous to add more indium at this time byusing the surface tension of the indium to form a “puddle” of moltenindium.

If tin is used as the bonding layer 60, the procedure described in thepreceding paragraph is used, but tin is used in place of indium and theresonator 26 and the crystal 30 are heated to approximately 240° C.Since pure tin melts at 231.9° C., the tin is not applied to theresonator 26 and the crystal 30 until both pieces have reached atemperature of approximately 240° C.

The resonator 26 and the piezoelectric crystal 30 are then pressedtogether so that the surfaces coated with indium or tin are in contactwith each other, thereby forming the transducer 14. The newly formedtransducer 14 is allowed to cool to room temperature so that the indiumor tin solidifies. Preferably, the solid bonding layer 60 has athickness “g” which is just sufficient to form a void free bond (i.e.the thinner the better). In the preferred embodiment, “g” isapproximately one mil (0.001 inches). Thicknesses up to about 0.01inches could be used, but the efficiency of acoustic transmission dropsoff when the thickness “g” is increased.

Preferably, the transducer 14 is allowed to cool with the piezoelectriccrystal 30 on top of the resonator 26 and the force of gravity holdingthe two pieces together. Alternatively, a weight can be placed on top ofthe piezoelectric crystal 30 to aide in the bonding of the indium.Another alternative is to place the newly formed transducer 14 in aclamping fixture. Once the transducer 14 has cooled to room temperature,any excess indium or tin that has seeped out from between thepiezoelectric crystal 30 and the resonator 26, is removed with a tool orother means.

FIG. 5 illustrates a preferred embodiment of an acoustic transducersystem 124 in which the resonator can be one of several chemically inertmaterials. These materials include sapphire, quartz, silicon carbide,silicon nitride, aluminum, stainless steel and ceramics. The transducersystem 124 shown in FIG. 5 is similar to the transducer 14 shown in FIG.2. However, several of the attachment layers used in the transducersystem 124 are different.

In FIG. 5, the acoustic transducer system 124 comprises a piezoelectriccrystal 130 attached to a resonator 134 by a bonding layer 138. Aplurality of attachment layers are disposed between the piezoelectriccrystal 130 and the resonator 134 to facilitate the attachment process.Specifically, a second wetting layer 142 is present adjacent to a frontsurface 146 of the bonding layer 138. A first wetting layer 150 ispresent adjacent to a back surface 154 of the bonding layer 138. A firstadhesion layer 158 is positioned between the first wetting layer 150 andthe piezoelectric crystal 130 to facilitate the mechanical adhesion ofthe bonding layer 138 to the crystal 130. The bonding layer 138comprises a solderable material such as indium or tin.

In the preferred embodiment, the first adhesion layer 158 comprises anapproximately 5000 Å thick layer of an alloy comprised of chrome and anickel copper alloy, such as the alloys marketed under the trademarksNickel 400™ or MONEL™. However, other materials and/or thicknesses couldalso be used as the first adhesion layer. 158. Nickel 400™ and MONEL™are copper nickel alloys comprised of 32% copper and 68% nickel.

Preferably, the wetting layers 142 and 150 comprise silver. The wettinglayers 142 and 150 each have a thickness of approximately 5000 Å.However, other metals and/or thicknesses could be used for the wettinglayers 142 and 150. The function of the wetting layers 142 and 150 is toprovide a wetting surface for the molten indium or tin, meaning that thelayers 142 and 150 help the bonding layer 138 adhere to the firstadhesion layer 158 and a second adhesion layer 162, respectively. It isthought that the silver in the wetting layers 142 and 150 forms an alloywith the indium or tin, thereby helping the bonding layer 138 adhere tothe adhesion layers 158 and 162. The transducer system 124 includes astep-region 195 in the wetting layer 142 which is exactly analogous tothe step-region 110 described previously with respect to FIG. 2.

In the preferred embodiment, the piezoelectric crystal 130 is identicalto the piezoelectric crystal 30 already described, and is comprised oflead zirconate titanate (PZT). However, many other piezoelectricmaterials such as barium titanate, quartz or polyvinylidene fluorideresin (PVDF), may be used as is well-known in the art. In the preferredembodiment, four rectangularly shaped PZT crystals are used in thetransducer 14 (shown in FIG. 6), and each PZT crystal is individuallyexcited. However, other numbers of the crystals 130 can be used,including between one and sixteen of the crystals 130, and other shapes,such as round crystals, could be used.

The second adhesion layer 162 separates the second wetting layer 142from the resonator 134. In the preferred embodiment, the adhesion layer162 comprises an approximately 5000 Å thick layer of an alloy comprisedof chrome and a nickel copper alloy, such as the alloys marketed underthe trademarks Nickel 400™ or MONEL™. However, other materials and/orthicknesses could also be used as the second adhesion layer 162.

The function of the first adhesion layer 158 is to form a strong bondbetween the bonding layer 138 and the piezoelectric crystal 130. Asnoted previously, the wetting layer 150 forms an alloy with the indiumor tin in the bonding layer 138, thereby permitting the adhesion layer158 to bond with the bonding layer 138. Similarly, the function of thesecond adhesion layer 162 is to form a strong bond between the bondinglayer 138 and the resonator 134. The wetting layer 142 forms an alloywith the indium in the bonding layer 138, thereby permitting theadhesion layer 162 to bond with the bonding layer 138. Additionally, thefirst adhesion layer 158 needs to be electrically conductive in order tocomplete the electrical path from the step region 195 to the surface ofthe piezoelectric crystal 130. Furthermore, the adhesion layers 158 and162 may prevent (block) the indium or tin in the bonding layer 138 fromreacting with the crystal 130 and/or the resonator 134, respectively.

An additional two layers are disposed on a back side of thepiezoelectric crystal 130 (i.e. on the side facing away from theresonator 134). Specifically, a third adhesion layer 169 and a metallayer 170 are positioned on the back side of the piezoelectric crystal130. The layers 169 and 170 cover substantially all of the surface areaof the back side of the crystal 130. In the preferred embodiment, thethird adhesion layer 169 comprises an approximately 5000 Å thick layerof an alloy comprised of chrome and a nickel copper alloy, such as thealloys marketed under the trademarks Nickel 400™ or MONEL™. However,other materials and/or thicknesses could also be used as the thirdadhesion layer 169. The function of the third adhesion layer 169 is topromote adhesion of the metal layer 170 to the crystal 130.

Preferably, the metal layer 170 comprises silver, although otherelectrically conductive metals such as nickel could also be used.Generally, the crystal 130 is obtained from commercial sources withoutthe layers 169 and 170. The layers 169 and 170 are then applied to thepiezoelectric crystal 130 using a sputtering technique such as physicalvapor deposition (PVD). The layer 170 functions as an electrode toconduct electricity from a set of the spring/button connectors shown inFIG. 1, so as to set up a voltage across the crystal 130. Since thethird adhesion layer 169 is also electrically conductive, both of thelayers 169 and 170 actually function as an electrode.

In the preferred embodiment, the bonding layer 138 comprises pure indium(99.99%) such as is commercially available from Arconium or Indalloy.However, indium alloys containing varying amounts of impurity metals canalso be used, albeit with less satisfactory results. The benefit ofusing indium and its alloys is that indium possesses excellent shearproperties that allow dissimilar materials with different coefficientsof expansion to be attached together and experience thermal cycling(i.e. expansion and contraction at different rates) without damage tothe attached materials or to the resonator 134. The higher the purity ofthe indium, the better the shear properties of the system 124 will be.If the components of the acoustic transducer system 124 have similarcoefficients of expansion, then less pure indium can be used becauseshear factors are less of a concern. Less pure indium (i.e. alloys ofindium) has a higher melting point then pure indium and thus may be ableto tolerate more heat. Similarly, when tin is used as the bonding layer138, it is preferable to use pure tin (99.99% tin). However, tincontaining varying amounts of impurities, including other metals, canalso be used.

Depending upon the requirements of a particular cleaning task, thecomposition of the resonator 134 is selected from a group of chemicallyinert materials. For example, inert materials that work well as theresonator 134 include sapphire, quartz, silicon carbide, siliconnitride, aluminum, stainless steel and ceramics. One purpose of theresonator 134 is to separate (isolate) the piezoelectric crystal 130from the fluid used in 20 the cleaning process, so that the fluid doesnot damage the crystal 130. Additionally, it is unacceptable for theresonator 134 to chemically react with the cleaning fluid. Thus, thematerial used as the resonator 134 is usually dictated, at least inpart, by the nature of the cleaning fluid. Sapphire is a desirablematerial for the resonator 134 when the items to be cleaned by themegasonic cleaning apparatus require parts per trillion purity. Forexample, semiconductor wafers require this type of purity. A hydrogenfluoride (HF) based cleaning fluid might be used in a cleaning processof this type for semiconductor wafers.

The resonator 134 must also be able to transmit the acoustic energygenerated by the crystal 130 into the fluid. Therefore, the acousticproperties of the resonator 134 are important. Generally, it isdesirable that the acoustic impedance of the resonator 134 be betweenthe acoustic impedance of the piezoelectric crystal 130 and the acousticimpedance of the cleaning fluid in the fluid chamber 190 (shown in FIG.6). Preferably, the closer the acoustic impedance of the resonator 134is the acoustic impedance of the cleaning fluid, the better.

In one preferred embodiment, the resonator 134 is a piece of syntheticsapphire (a single crystal substrate of Al₂O₃). Preferably, the sapphireis high grade having a designation of 99.999% (5 9s+purity). Whensynthetic sapphire is used as. the resonator 134, the thickness “v”,illustrated in FIG. 5 is approximately six millimeters. It should benoted that other forms of sapphire could be used as the resonator 134,such as rubies or emeralds. However, for practical reasons such as costand purity, synthetic sapphire is preferred. Additionally, other valuesfor the thickness “v” can be used.

In the preferred embodiment, the thickness “v” of the resonator 134 is amultiple of one-half of the wavelength of the acoustic energy in theresonator 134, so as to minimize reflectance problems. For example, “v”is approximately six millimeters for sapphire and acoustic energy ofapproximately 925 KHz. The wavelength of acoustic energy in theresonator 134 is governed by the relationship shown in equation 1 below:λ_(1/2) =v _(L)/2f   (1)where,

-   -   v_(L)=the velocity of sound in the resonator 134 (in mm/msec),    -   f=the natural frequency of the piezoelectric crystal 130 (in        MHz)    -   λ_(1/2)=one half the wavelength of acoustic energy in the        resonator 134.

From equation 1, it follows that when the composition of the resonatorchanges or when the natural resonance frequency of the crystal 130changes, the ideal thickness of the resonator 134 will change.Therefore, in all of the examples discussed herein, a thickness “v”which is a multiple of one-half of the wavelength λ could be used.

In another preferred embodiment, the resonator 134 is a piece of quartz(SiO₂—synthetic fused quartz). Preferably, the quartz has a purity of99.999% (5 9s+purity). When quartz is used as the resonator 134, thethickness “v”, illustrated in FIG. 5 is approximately three to sixmillimeters.

In another preferred embodiment, the resonator 134 is a piece of siliconcarbide (SiC). Preferably, the silicon carbide has a purity of 99.999%(5 9s+purity, semiconductor grade). When silicon carbide is used as theresonator 134, the thickness “v”, illustrated in FIG. 5 is approximatelysix millimeters.

In another preferred embodiment, the resonator 134 is a piece of siliconnitride (Si₃N₄). Preferably, the silicon nitride has a purity of 99.999%(5 9s+purity, semiconductor grade). When silicon nitride is used as theresonator 134, the thickness “v”, illustrated in FIG. 5 is approximatelysix millimeters.

In another preferred embodiment, the resonator 134 is a piece of ceramicmaterial. In this application, the term ceramic means alumina (Al₂O₃)compounds such as the material supplied by the Coors Ceramics Companyunder the designation Coors AD-998. Preferably, the ceramic material hasa purity of at least 99.8% Al₂O₃. When ceramic material is used as theresonator 134, the thickness “v”, illustrated in FIG. 5 is approximatelysix millimeters.

The acoustic transducer system 124 illustrated in FIG. 5 is prepared bythe following method: Assuming that the resonator 134 is sapphire, thesurface of the sapphire that will be adjacent to the adhesion layer 162is cleaned by abrasive blasting or chemical or sputter etching. Theadhesion layer 162 is then deposited on the resonator 134 using aphysical vapor deposition (“PVD”) technique, such as argon sputtering.More specifically, the chrome and nickel copper alloy (e.g. Nickel 400™or MONEL™) that comprise the layer 162 are co-sputtered onto to theresonator 134 so that the layer 162 is comprised of approximately 50%chrome and 50% nickel copper alloy. The wetting (silver) layer 142 isthen deposited on the adhesion layer 162 using argon sputtering. Aplating technique could also be used in this step.

The piezoelectric crystal 130 is preferably purchased without anyelectrode layers deposited on its surfaces. The third adhesion layer 169is then deposited on the crystal 130 using a PVD technique, such asargon sputtering. More specifically, the chrome and nickel copper alloythat comprise the layer 169 are co-sputtered onto to the crystal 130 sothat the layer 169 is comprised of approximately 50% chrome and 50%nickel copper alloy (e.g. Nickel 400™ or MONEL™). The electrode (silver)layer 170 is then deposited on the adhesion layer 169 using argonsputtering. A plating technique could also be used in this step.

Similarly, the first adhesion layer 158 is deposited on the oppositeface of the crystal 130 from the third adhesion layer 169 using a PVDtechnique like argon sputtering. More specifically, the chrome andnickel copper alloy that comprise the layer 158 are co-sputtered onto tothe crystal 130 so that the layer 158 is comprised of approximately 50%chrome and 50% nickel copper alloy. The wetting (silver) layer 150 isthen deposited on the adhesion layer 158 using argon sputtering. Aplating technique could also be used in this step.

If indium is used as the bonding layer 138, the resonator 134 and thepiezoelectric crystal 130 are both heated to approximately 200° C.,preferably by placing the resonator 134 and the crystal 130 on a heatedsurface such as a hot-plate. When both pieces have reached a temperatureof greater than 160° C., solid indium is rubbed on the surfaces of theresonator 134 and the crystal 130 which are to be attached. Since pureindium melts at approximately 157° C., the solid indium liquefies whenit is applied to the hot surfaces, thereby wetting the surfaces withindium. It is sometimes advantageous to add more indium at this time byusing the surface tension of the indium to form a “puddle” of moltenindium.

If tin is used as the bonding layer 138, the procedure described in thepreceding paragraph is used, but tin is used in place of indium and theresonator 134 and the crystal 130 are heated to approximately 240° C.Since pure tin melts at 231.9° C., the tin is not applied to theresonator 26 and the crystal 30 until both pieces have reached atemperature of approximately 240° C.

The resonator 134 and the piezoelectric crystal 130 are then pressedtogether so that the surfaces coated with indium or tin are in contactwith each other, thereby forming the transducer system 124. The newlyformed transducer system 124 is allowed to cool to room temperature sothat the indium or tin solidifies. Preferably, the bonding layer 138 hasa thickness “g” which is just sufficient to form a void free bond. Inthe preferred embodiment, “g” is approximately one mil (0.001 inches).It is thought that the thickness “g” should be as small as possible inorder to maximize the acoustic transmission, so thicknesses less thanone mil might be even more preferable. Thicknesses up to about 0.01inches could be used, but the efficiency of acoustic transmission dropsoff when the thickness “g” is increased.

Preferably, the transducer system 124 is allowed to cool with thepiezoelectric crystal 130 on top of the resonator 134 and the force ofgravity holding the two pieces together. Alternatively, a weight can beplaced on top of the piezoelectric crystal 130 to aide in the bonding ofthe indium or tin. Another alternative is to place the newly formedtransducer system 124 in a clamping fixture.

Once the transducer system 124 has cooled to room temperature, anyexcess indium or tin that has seeped out from between the piezoelectriccrystal 130 and the resonator 134, is removed with a tool or othermeans.

FIG. 6 illustrates a megasonic cleaning system 180 that utilizes theacoustic transducer system 124 (or the acoustic transducer 14). Thecleaning solution is contained within a tank 184. In the preferredembodiment, the tank 184 is square-shaped and has four vertical sides188. The resonator 134 forms a detachable bottom surface of the tank184. Other shapes can be used for the tank 184, and in otherembodiments, the resonator 134 can be a detachable portion of the bottomsurface of the tank 184.

A fluid chamber 190 is the open region circumscribed by the sides 188.Since the sides 188 do not cover the top or bottom surfaces of the tank184, the sides 188 are said to partially surround the fluid chamber 190.The fluid chamber 190 holds the cleaning solution so the walls 188 andthe resonator 134 must make a fluid tight fit to prevent leakage. Theresonator 134 has an interface surface 191 which abuts the fluid chamber190 so that the interface surface 134 is in contact with at least someof the cleaning solution when cleaning solution is present in the fluidchamber 190. Obviously, the interface surface 191 is only in contactwith the cleaning solution directly adjacent to the surface 191 at anypoint in time.

In the preferred embodiment shown in FIG. 6, four piezoelectric crystals130 are used. In a typical preferred embodiment, each of the crystals isa rectangle having dimensions of 1 inch (width)×6 inch (length “k” inFIG. 5)×0.10 inch (thickness “s” in FIG. 5). Since the natural frequencyof the crystal changes with thickness, reducing the thickness will causethe natural frequency of the crystal to be higher. As was indicatedpreviously, other numbers of crystals can be used, other shapes for thecrystals can be used and the crystals can have other dimensions such as1.25×7×0.10 inches or 1.5×8×0.10 inches. Each of the crystals 130 areattached to the resonator 134 by the plurality of layers describedpreviously with respect to FIG. 5. A gap 192 exists between eachadjacent crystal 130 to prevent coupling of the crystals.

The power for driving the crystals 130 is provided by a radiofrequency(RF) generator 194 (shown in FIG. 7). The electrical connections betweenthe RF generator 194 and the crystals 130 are provided by the pluralityof first spring/button connectors 38 and the plurality of secondspring/button connectors 42, as was explained previously with respect toFIGS. 1 and 3. The plurality of second spring/button connectors 42provide the active connection to the RF generator 194 and the pluralityof first spring/button connectors 38 provide the ground connection tothe RF generator 194.

The transducer system 124 includes the step-region 195 (shown in FIG. 5)which is exactly analogous to the step-region 110 described previouslywith respect to FIG. 2. The step region 195 is a region on the secondwetting layer 142 that can be contacted by the upper buttons 90 of thespring/button connectors 38. Since all of the layers between the secondwetting layer 142 and the crystal 130 are electrically conductive (i.e.the layers 138, 150 and 158), contact with the step region 195 isequivalent to contact with the surface front surface of the crystal 130.The upper buttons 98 of the spring/button connectors 42 make electricalcontact with the metal layer 170 to complete the circuit for driving thePZT crystal 130. This circuit is represented schematically in FIG. 7.

Referring to FIG. 6, the printed circuit board (PCB) 34 and thepiezoelectric crystal 130 are positioned in a cavity 46 and aresurrounded by the housing 22 as was described previously with respect toFIG. 1. A plurality of items 196 to be cleaned are inserted through thetop of the tank 184.

The acoustic transducer system 124 (illustrated in FIG. 5) functions asdescribed below. It should be noted that the transducer 14 (illustratedin FIG. 2) works in the same manner as the acoustic transducer system124. However, for the sake of brevity, the components of the system 124are referenced in this discussion.

A radiofrequency (RF) voltage supplied by the RF generator 194 creates apotential difference across the piezoelectric crystal 130. Since this isan AC voltage, the crystal 130 expands and contracts at the frequency ofthe RF voltage and emits acoustic energy at this frequency. Preferably,the RF voltage applied to the crystal 130 has a frequency ofapproximately 925 KHZ. However, RF voltages in the frequency range ofapproximately 0.4 to 2.0 MHZ can be used with the system 124, dependingon the thickness and natural frequency of the crystal 130. A 1000 wattRF generator such as is commercially available from Dressler Industriesof Strohlberg, Germany is suitable as the RF generator 194.

In the preferred embodiment, only one of the crystals 130 is driven bythe RF generator at a given time. This is because each of the crystals130 have different natural frequencies. In the preferred embodiment, thenatural frequency of each crystal 130 is determined and stored insoftware. The RF generator then drives the first crystal at the naturalfrequency indicated by the software for the first crystal. After aperiod of time (e.g. one millisecond), the RF generator 194 stopsdriving the first crystal and begins driving the second crystal at thenatural frequency indicated by the software for the second crystal 130.This process is repeated for each of the plurality of crystals.Alternatively, the natural frequencies for the various crystals 130 canbe approximately matched by adjusting the geometry of the crystals, andthen driving all of the crystals 130 simultaneously.

Most of the acoustic energy is transmitted through all of the layers ofthe system 124 disposed between the crystal 130 and the resonator 134,and is delivered into the cleaning fluid. However, some of the acousticenergy generated by the piezoelectric crystal 130 is reflected by someor all of these layers. This reflected energy can cause the layers toheat up, especially as the power to the crystal is increased.

In the present invention, the bonding layer 138 has an acousticimpedance that is higher than the acoustic impedance of other attachmentsubstances, such as epoxy. This reduces the amount of reflected acousticenergy between the resonator 134 and the bonding layer 138. This createstwo advantages in the present invention. First, less heat is generatedin the transducer system, thereby allowing more RF power to be appliedto the piezoelectric crystal 130. For example, in the transducer systemillustrated in FIG. 5, 25 to 30 watts/cm² can be applied to the crystal130 (for an individually excited crystal) without external cooling.Additionally, the system 124 can be run in a continuous mode withoutcooling (e.g. 30 minutes to 24 hours or more), thereby allowing bettercleaning to be achieved. In contrast, prior art systems useapproximately 7 to 8 watts/cm², without external cooling. Prior artmegasonic cleaning systems that operate at powers higher than 7 to 8watts/cm² in a continuous mode require external cooling of thetransducer.

Second, in the present invention, the reduced reflectance allows morepower to be delivered into the fluid, thereby reducing the amount oftime required in a cleaning cycle. For example, in the prior art, acleaning cycle for sub 0.5 micron particles generally requires fifteenminutes of cleaning time. With the present invention, this time isreduced to less than one minute for many applications. In general, theuse of the bonding layer 138 permits at least 90 to 98% of the acousticenergy generated by the piezoelectric crystal 130 to be transmitted intothe cleaning fluid when the total power inputted to the piezoelectriccrystal 130 is in the range of 400 to 1000 watts (e.g. 50 watts/cm² fora crystal 130 having an area of 20 cm²). In the preferred embodiment,the bonding layer 138 attenuates the acoustic energy that is transmittedinto the volume of cleaning fluid by no more than approximately 0.5 dB.It is believed that the system 124 can be used with power as high as5000 watts. In general, the application of higher power levels to thepiezoelectric crystal 130 results in faster cleaning times. It may alsolead to more thorough cleaning.

Table 1 below indicates the power levels that can be utilized when theindicated materials are used as the resonator 134 in the system 124. Theinput wattage (effective power) is defined as the forward power into thecrystal 130 minus the reflected power back into the RF generator 194. Asindicated above, the system 124 allows at least approximately 90 to 98%of the input wattage to be transmitted into the cleaning solution. TABLE1 Resonator Input Wattage/cm² Quartz 12.5 watts/cm² Silicon carbide orsilicon nitride   20 watts/cm² Stainless steel   25 watts/cm² Ceramic  40 watts/cm² Sapphire   50 watts/cm²

FIG. 8 illustrates a megasonic cleaning system 200 comprised of a tank204, which is a one-piece vessel, and an acoustic transducer system 206.The system 200 is similar to the system 180 (shown in FIG. 6), andelements in the system 200 that are identical to corresponding elementsin the system 180 are referred to with the same reference numerals. Thetank 204 is comprised entirely of a single material such as quartz,silicon carbide, silicon nitride, ceramics, aluminum or stainless steel.Sapphire could also be used, but would be extremely expensive.Representative specifications for these materials, including source,purity and thickness “v”, were described previously with respect to FIG.5. When the constraints of equation 1 are taken into consideration, thepreferred thicknesses of the side 208 or the bottom 216 to which thecrystals 130 are attached are three, six or nine millimeters for quartzand stainless steel tanks.

In the preferred embodiment, the tank 204 is square-shaped and has fourvertical sides 208, having corners 212 which are preferably curved. Abottom 216 is positioned perpendicular to the four sides 208, so thatthe tank 204 forms a hollow cube with the top of the tank 204 beingopen. A fluid chamber 217 (analogous to the fluid chamber 190 shown inFIG. 5) for accepting the cleaning fluid is the open regioncircumscribed by the sides 208 and the bottom 216. In FIG. 8, a portionof the sides 208 have been cut away to illustrate the bottom 216, but inan actual tank, no such cut away section exists. In a representativetank, the sides 208 have a height “h” of approximately forty centimeters(cm), a length “l” of approximately thirty cm, and a thickness “t” ofapproximately six millimeters (mm). Other dimensions or shapes can beused for the tank 184, such as a rectangle, but the symmetry of a squaretank is preferred. In alternate embodiments, the sides 208 may be angledto focus the acoustic energy.

Preferably, the tank 204 comprises a single continuous piece of thematerial from which it is constructed so that there are no mechanicaljoints where the sides or bottom of the tank come together. For example,where the tank 204 comprises quartz, the sides 208 and bottom 216 arecontinuous, without a seam or joint between them, so that the tank 204comprises a single continuous piece of quartz. The advantage in using atank comprised of a single continuous piece of material is that thereare no cracks or other voids at the interfaces between the sides 208 andbetween the sides 208 and the bottom 216, where particles or othercontaminates could accumulate. Thus, the system 200 is used in cleaningsituations where an extremely high degree of cleanliness is required,even by integrated circuit manufacturing standards. The single piececonstruction also allows a quartz tank to withstand higher cleaningtemperatures.

In the system 200, the acoustic transducer system 206 is attacheddirectly to the bottom 216, or to one of the sides 208, of the tank 204,using the bonding layer 138 (shown in FIG. 5). Therefore, in the system200, the region of the side 208, or the region of the bottom 216, towhich the piezoelectric crystals 130 are attached, functions as theresonator 134 (shown FIG. 5). This attachment of the crystal or crystals130 directly to the tank 204 can be accomplished using the processes andmaterials that were described previously with respect to FIG. 5, or theprocesses and materials that were described with respect to FIG. 2.Specifically, the layers 158, 150, 138, 142 and 162 are positionedbetween the crystal 130 and the resonator 134. However, since the tank204 may be too large to fit into a sputtering apparatus or into anevaporator, a third attachment process is described below that uses adifferent attachment process and materials.

FIG. 9 illustrates the layers used in the third attachment process.Layers that are identical to the layers described previously withrespect to FIG. 5 are identified with the same reference numerals usedin FIG. 5. Inspection of FIG. 9 shows that the bottom 216 of the tank204 is now in the position of the resonator 124 shown in FIG. 5.Additionally, a combination layer 220 has replaced the second adhesionlayer 162 and the second wetting layer 142 shown in FIG. 5. Thecombination layer 220 is positioned between the bottom 216 and thebonding layer 138. It should be appreciated that the crystal (orcrystals) 130 could be attached to one of the sides 208 instead of tothe bottom 216. In the description below, attachment to the bottom 216is described, but the same procedure can be used for attachment to oneof the sides 208. As was described previously with respect to FIG. 5,the bonding layer 138 can be comprised of either indium or tin.

Typically, the combination layer 220 is applied to a region of thebottom 216 to which the crystal or crystals 130 will be attached. Thisregion does not cover the entire surface area of the bottom 216, but itis preferably larger than the surface area of the crystal or crystals130. This allows a portion of the combination layer 220 to function as astep region 224. The step region 224 functions like the step regions 195and 110 described previously with respect to FIGS. 5 and 2,respectively.

In the preferred embodiment, the combination layer 220 is a conductivesilver emulsion (paste) that is applied to the bottom 216. An acceptableemulsion is the commercially available product referred to as the 2617Dlow temperature silver conductor, available from EMCA-REMAX Products, ofMontgomeryville, Pa. The layer 220 is applied directly to the bottom 216using screen printing techniques, thereby avoiding the need to place thetank 204 inside of a sputtering or evaporation apparatus as would beneeded if the layers 162 and 142 were used.

In the preferred embodiment, the layer 220 is applied as follows: Theoutside surface of the bottom 216 is cleaned, such as by bead blasting.A 325 mesh stainless steel screen is placed over the outside surface ofthe bottom 216 and an approximately 0.5 mil thick layer of the 2617D lowtemperature silver conductor paste is coated over the screen. The screenis then removed from the bottom 216 and the bottom 216 is allowed to airdry at room temperature for two to five minutes. The entire tank 204 isthen placed in a convection oven and dried at approximately 150° C. forapproximately ten minutes. This process results in the tank 204 havingthe layer 220 firmly coated on the outside surface of the bottom 216.The layer 220 has a thickness of approximately ten to twenty-fivemicrons (10-25×10⁻⁶ m).

The piezoelectric crystal (or crystals) 130 is then attached to thebottom 216 with the bonding layer 138 using the technique describedpreviously with respect to FIG. 5. Specifically, the piezoelectriccrystal 130 is preferably purchased without any electrode layersdeposited on its surfaces. The third adhesion layer 169 is thendeposited on the crystal 130 using a PVD technique, such as argonsputtering. More specifically, the chrome and nickel copper alloy thatcomprise the layer 169 are co-sputtered onto to the crystal 130 so thatthe layer 169 is comprised of approximately 50% chrome and 50% nickelcopper alloy (e.g. Nickel 400™ or MONEL™). The electrode (silver) layer170 is then deposited on the adhesion layer 169 using argon sputtering.A plating technique could also be used in this step.

Similarly, the first adhesion layer 158 is deposited on the oppositeface of the crystal 130 from the third adhesion layer 169 using a PVDtechnique like argon sputtering. More specifically, the chrome andnickel copper alloy that comprise the layer 158 are co-sputtered onto tothe crystal 130 so that the layer 158 is comprised of approximately 50%chrome and 50% nickel copper alloy. The wetting (silver) layer 150 isthen deposited on the adhesion layer 158 using argon sputtering. Aplating technique could also be used in this step.

The bottom 216 and the piezoelectric crystal 130 are attached asfollows: If indium is used as the bonding layer 138, the bottom 216 andthe crystal 130 are both heated to approximately 200° C., preferably byplacing the bottom 216 and the crystal 130 on a heated surface such as ahot-plate. When both pieces have reached a temperature of greater than160° C., solid indium is rubbed on the surfaces of the bottom 216 andthe crystal 130 which are to be attached (i.e. the indium is rubbed onthe layer 220 and on the first wetting layer 150). Since pure indiummelts at approximately 157° C., the solid indium liquefies when it isapplied to the hot surfaces, thereby wetting the surfaces with indium.It is sometimes advantageous to add more indium at this time by usingthe surface tension of the indium to form a “puddle” of molten indium.

If tin is used as the bonding layer 138, the procedure described in thepreceding paragraph is used, but tin is used in place of indium, and thebottom 216 and the crystal 130 are heated to approximately 240° C. Sincepure tin melts at 231.9° C., the tin is not applied to the bottom 216and the crystal 130 until both pieces have reached a temperature ofapproximately 240° C.

The tank 204 (bottom 216) and the piezoelectric crystal 130 are thenpressed together so that the surfaces coated with indium or tin are incontact with each other, thereby forming the transducer system 200. Thenewly formed transducer system 200 is allowed to cool to roomtemperature so that the indium or tin solidifies. Preferably, thebonding layer 138 has a thickness “g” which is just sufficient to form avoid free bond. In the preferred embodiment, “g” is approximately onemil (0.001 inches). It is thought that the thickness “g” should be assmall as possible in order to maximize the acoustic transmission, sothicknesses less than one mil might be even more preferable. Thicknessesup to about 0.01 inches could be used, but the efficiency of acoustictransmission drops off when the thickness “g” is increased.

Preferably, the transducer system 200 is allowed to cool with thepiezoelectric crystal 130 on top of the bottom 216 and the force ofgravity holding the two pieces together. Alternatively, a weight can beplaced on top of the piezoelectric crystal 130 to aide in the bonding ofthe indium or tin. Another alternative is to place the newly formedtransducer system 200 in a clamping fixture.

The transducer system 200 functions in a similar manner to the system124 described previously with respect to FIG. 5. The only difference isthat the resonator 134 of FIG. 5 is replaced with the region of the tank204 to which the crystals 130 are attached (i.e. either a region of thebottom 216 or a region of the sides 208).

As was mentioned previously, the combination layer 220 can be replacedby the second adhesion layer 162 and the second wetting layer 142.Suitable materials and processes for utilizing the second adhesion layer162 and the second wetting layer 142 were described previously withrespect to FIG. 5. For example, the second adhesion layer 162 may becomprised of chromium, copper and nickel and is positioned in contactwith a surface of the tank. The second wetting layer 142 may becomprised of silver and is positioned between the second adhesion layer162 and the bonding layer 138 for helping the bonding layer bond to thesecond adhesion layer 162. Similarly, the combination layer 220 can bereplaced by the blocking adhesion layer 84 and the first metal layer 64described previously with respect to FIG. 2. The drawback to both ofthese embodiments is that the whole tank 204 may have to be placed in aphysical vapor deposition (PVD) chamber so that the layers 162 and 142,or 84 and 64, can be sputtered onto the tank 204.

FIG. 10 illustrates an embodiment of an acoustic transducer system 230in which the combination layer 220 is positioned between a resonator 234and a bonding layer 238. The system 230 is similar to the acoustictransducer system 206 shown in FIG. 9, and layers shown in FIG. 10 thatare identical to the layers described previously with respect to FIG. 9are identified with the same reference numerals used in FIG. 9.

In FIG. 10, the resonator 234 can be a piece of material that is notoriginally part of the cleaning tank such as the resonator 134 shown inFIGS. 5 and 6, or the resonator 26 shown in FIGS. 1 and 2. The resonator234 is comprised of the same materials described previously with respectto the resonators 26 and 134. Specifically, the resonator 234 iscomprised entirely of a single material such as quartz, sapphire,silicon carbide, silicon nitride, aluminum, ceramics or stainless steel.In general, when aluminum is used as the resonator 234, the surface ofthe resonator 234 that is in contact with the cleaning fluid, is coatedor modified to reduce the reactivity of the aluminum with the cleaningfluid. Coatings that can be used include Teflon or Kynar. Modificationsof the aluminum surface that can be used include hard coat anodizing(i.e. anodization with sulfuiric acid) or other types of anodization.These same considerations about aluminum apply when using aluminum asthe resonators 26 or 134, shown in FIGS. 2 and 5, or as the bottom 216shown in FIG. 9.

In FIG. 10, the bonding layer 238 comprises either indium or tin. If thebonding layer 238 comprises indium, the system 230 is constructed withthe same materials that were described previously with respect to FIGS.2, 5 and 9. When tin is used as the bonding layer 238, it is preferableto use relatively pure tin, such as 99.99% pure tin, and the system 230is constructed as follows.

The combination layer 220 is applied to the resonator 234 as wasdescribed previously with respect to FIG. 9. Specifically, a surface ofthe resonator 234 is cleaned, such as by bead blasting. A stainlesssteel screen (preferably 325 mesh) is placed over the cleaned surface ofthe resonator 234 and an approximately 0.5 mil thick layer of silverconductor paste (preferably 2617D low temperature silver conductorpaste) is coated over the screen and the paste is forced through thescreen. The screen is then removed and the resonator 234 is allowed toair dry at room temperature for two to five minutes. The resonator 234is then placed in a convection oven and dried at approximately 150° C.for approximately ten minutes. Note that if the resonator is an integralpart of the tank, then the entire tank is placed in the oven, as wasdescribed previously with respect to FIG. 9.

The piezoelectric crystal (or crystals) 130 is then attached to theresonator 234 with the bonding layer 238 using a similar technique tothe one described previously with respect to FIG. 9. Specifically, thepiezoelectric crystal 130 is preferably purchased without any electrodelayers deposited on its surfaces. The third adhesion layer 169 and themetal layer 170 are then applied to the crystal 130 as was describedpreviously with respect to FIG. 9. Similarly, the first adhesion layer158 is deposited on the opposite face of the crystal 130 from the thirdadhesion layer 169, and the wetting (silver) layer 150 is then depositedon the adhesion layer 158.

The resonator 234 and the crystal 130 are attached using the techniquedescribed previously with respect to FIGS. 2, 5 and 9. Specifically,when tin is used as the bonding layer 238, the resonator 234 and thepiezoelectric crystal 130 are both heated to approximately 240° C.,preferably by placing the resonator 234 and the crystal 130 on a heatedsurface such as a hot-plate. When both pieces have reached a temperatureof approximately 240° C., solid tin is rubbed on the surfaces of theresonator 234 and the crystal 130 which are to be attached (i.e. the tinis rubbed on the layer 220 and on the first wetting layer 150). Sincepure tin melts at approximately 231.9° C., the solid tin liquefies whenit is applied to the hot surfaces, thereby wetting the surfaces withtin. It is sometimes advantageous to add more tin at this time by usingthe surface tension of the tin to form a “puddle” of molten tin.

When indium is used as the bonding layer 238, the procedure used in thepreceding paragraph is used, but indium is used in place of tin, and theresonator 234 and the crystal 130 are heated to approximately 200° C.

The resonator 234 and the piezoelectric crystal 130 are then pressedtogether so that the surfaces coated with tin or indium are in contactwith each other, thereby forming the transducer system 230. The newlyformed transducer system 230 is allowed to cool to room temperature sothat the tin or indium solidifies. Preferably, the bonding layer 238 hasa thickness “g” which is just sufficient to form a void free bond. Inthe preferred embodiment, “g” is approximately one mil (0.001 inches).It is thought that the thickness “g” should be as small as possible inorder to maximize the acoustic transmission, so thicknesses less thanone mil might be even more preferable. Thicknesses up to about 0.01inches could be used, but the efficiency of acoustic transmission dropsoff when the thickness “g” is increased.

Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

1. A transducer comprising: an acoustic energy generating means forgenerating acoustic energy; a resonator for transmitting the acousticenergy into a volume of liquid, the resonator being adapted forpositioning between the acoustic energy generating means and the volumeof liquid; and a bonding layer comprised of indium or tin positionedbetween the resonator and the acoustic energy generating means forattaching the acoustic energy generating means to the resonator.
 2. Thetransducer of claim 1 wherein the acoustic energy generating meanscomprises a piezoelectric crystal.
 3. The transducer of claim 1 whereinthe acoustic energy has a frequency greater than 0.4 MHz.
 4. Thetransducer of claim 1 wherein the resonator comprises a materialselected from the group consisting of quartz, sapphire, silicon carbide,silicon nitride, aluminum, ceramics and stainless steel.
 5. Thetransducer of claim 1 wherein the bonding layer comprises indium havinga purity of at least 99.99%.
 6. The transducer of claim 1 wherein thebonding layer comprises tin having a purity of at least 99.99%.
 7. Acleaning system comprising: a container having a fluid chamber forholding a volume of cleaning solution; a resonator having an interfacesurface which abuts the fluid chamber so that the interface surface isin contact with at least some of the volume of cleaning solution whenthe volume of cleaning solution is present in the fluid chamber; anacoustic energy generating means for generating acoustic energy; and abonding layer comprised of indium or tin positioned between theresonator and the acoustic energy generating means for attaching theacoustic energy generating means to the resonator.
 8. The cleaningsystem of claim 7 wherein the resonator is part of the container.
 9. Thecleaning system of claim 7 wherein the resonator comprises a materialselected from the group consisting of quartz, sapphire, silicon carbide,silicon nitride, aluminum, ceramics and stainless steel.
 10. Thecleaning system of claim 7 wherein the acoustic energy generating meanscomprises a piezoelectric crystal.
 11. The cleaning system of claim 7wherein the acoustic energy has a frequency greater than 0.4 MHz. 12.The cleaning system of claim 7 wherein the bonding layer comprisesindium having a purity of at least 99.99%.
 13. The cleaning system ofclaim 7 wherein the bonding layer comprises tin having a purity of atleast 99.99%.
 14. A transducer comprising: an acoustic energy generatingmeans for generating acoustic energy; a resonator for transmitting theacoustic energy into a volume of cleaning fluid and adapted forpositioning between the acoustic energy generating means and the volumeof cleaning fluid; a bonding layer comprised of tin or indium positionedbetween the resonator and the acoustic energy generating means forattaching the acoustic energy generating means to the resonator; and acombination layer positioned between the bonding layer and theresonator, the combination layer functioning at least as a wetting layerto facilitate attachment of the bonding layer to the resonator.
 15. Thetransducer of claim 14 wherein the combination layer comprises a silveremulsion that is applied to the resonator using a screen process. 16.The transducer of claim 14 wherein the resonator comprises a materialselected from the group consisting of quartz, sapphire, silicon carbide,silicon nitride, aluminum, ceramics and stainless steel.
 17. Thetransducer of claim 14 wherein the acoustic energy generating meanscomprises a piezoelectric crystal.
 18. The transducer of claim 14wherein the acoustic energy has a frequency greater than 0.4 MHz. 19.The transducer of claim 14 wherein the bonding layer comprises indiumhaving a purity of at least 99.99%.
 20. The transducer of claim 14wherein the bonding layer comprises tin having a purity of at least99.99%.